1. Field of the Invention
The present invention relates to a liquid crystal display device.
2. Description of the Related Art
As the applications of liquid crystal devices to TV increase, the demand for liquid crystal display devices which respond at a high-speed to handle moving pictures is progressively increasing. Some high-speed response liquid crystal display devices use ferroelectric liquid crystals. But there are many problems with ferroelectric liquid crystals, including difficulties in maintaining a stable layer structure.
Modes for implementing a high-speed response using a nematic liquid crystal include an OCB (Optically Compensated Bend) mode and an OCS (Optically Compensated Splay) mode. According to these modes, the rubbing directions of both substrates are the same, and the alignment of liquid crystal molecules is formed such that the splay state becomes a lower energy state in the horizontal alignment and the bend state becomes a lower energy state in the vertical alignment, and these alignment states are each reversed by applying a predetermined voltage Vcr or higher. In other words, if liquid crystal molecules are aligned horizontally when voltage is not applied, the bend (OCB) state is implemented by applying voltage, and if liquid crystal molecules are aligned vertically when voltage is not applied, the splay (OCS) state is implemented by applying voltage. In these modes, if the liquid crystal molecules have been maintained to be OCB or OCS, the alignment change speed of the liquid crystal molecules thereafter can be made large, and therefore the response time of the display can be decreased to be extremely short.
In this way, OCB and OCS modes have a feature where the response time is very short in the change of all the grayscales, and exhibit high expectations in applications to TV. However, these alignment states return to their original alignment states where the liquid crystal molecules are stable in energy, if the voltage is lower than Vcr. From this original alignment, the alignment change speed of the liquid crystal molecules decreases, and the response time for a display cannot be decreased.
Therefore a voltage not less than a predetermined voltage Vcr must be applied continuously as long as the liquid crystal display device is in use, so that alignment does not return to the original alignment. Otherwise, alignment must be quickly shifted to the required alignment when the display device is turned ON.
A method to solve this problem is to add a photo-polymerizable monomer to the liquid crystal, and apply a voltage not less than Vcr in order to change the alignment state of the liquid crystal molecules (hereafter changing the alignment state may be called “alignment transition”), then fix the alignment state by irradiating ultraviolet rays (e.g. Japanese Patent Application Laid-Open No. 2003-43474 (Claims)).
This method will be described with reference to FIGS. 1A, 1B and 1C. FIG. 1A is a cross-sectional view of a liquid crystal display panel viewed from the side when a pair of vertical alignment control films 2 are formed on a pair of substrates 1, and have been rubbed so that the rubbing directions are parallel to each other, between which a liquid crystal composition comprising a liquid crystal 3 having a negative dielectric constant anisotropy and a polymerizable compound 4 which can be polymerized by light, heat or a combination thereof is filled. In FIG. 1A, the arrows indicate the rubbing direction. In this case, voltage is not applied between the substrates 1. In this state, the liquid crystal molecules 3 are in the bend state, as shown in FIG. 1A.
If a voltage not lower than Vcr is applied in this state, the liquid crystal molecules 3 change to the splay state, as shown in FIG. 1B. If ultraviolet rays are irradiated, for example, as shown in FIG. 1C, while maintaining this state (that is, while voltage is continuously applied), then resin films 5 made of the polymer are formed, and the alignment state of the liquid crystal molecules 3 is fixed.
FIGS. 2A, 2B and 2C show the case of liquid crystals having a positive dielectric constant anisotropy. FIG. 2A is a cross-sectional view of a liquid crystal display panel viewed from the side when a pair of horizontal alignment control films 2 are formed on a pair of substrates 1, and have been rubbed so that the rubbing directions are parallel to each other, between which a liquid crystal composition comprising a liquid crystal 3 having a positive dielectric constant anisotropy, and a polymerizable compound 4 which can be polymerized by light, heat or a combination thereof, is filled. In FIG. 2A, the arrows indicate the rubbing direction. In this case, voltage is not applied between the substrates 1. And in this state, the liquid crystal molecules 3 are in the splay state, as shown in FIG. 2A.
If a voltage not lower than Vcr is applied in this state, the liquid crystal molecules 3 change to the bend state, as shown in FIG. 2B. If ultraviolet rays are irradiated, for example, as shown in FIG. 2C, while maintaining this state (that is, while voltage is continuously applied), then resin films 5 made of the polymer are formed, and the alignment state of the liquid crystal molecules 3 is fixed.
However, a large quantity of monomers must be added to fix the alignment after the alignment transition, and the problems here include phenomena that monomers do not dissolve sufficiently into the liquid crystal, and coagulation is generated at polymerization, with the result that fixing the alignment after the transition without dropping the display quality is difficult. Also with this method, the range where retardation can be changed is small, and the change of transmittance with respect to the change of the applied voltage is small.